1. Field of the Invention
The present invention relates generally to linear position transmitters that utilize linear magnetostrictive wire to communicate the position of a magnet displaceable along the wire. The invention relates more specifically to an improved linear position transducer incorporating a magnetostrictive wire within which a torsional strain is detected by an improved piezoelectric sensor element which may be placed in an improved offset housing configuration for the transmitter.
2. Description of the Prior Art
Many state of the art systems for measuring liquid levels in storage tanks, as well as systems for measuring linear displacements for machine tools and the like, utilize a movable permanent magnet float or position indicator that surrounds a linearly oriented magnetostrictive wire. The use of the electromagnetic phenomenon of magnetostriction in such applications has long been recognized. Typically the position of the permanent magnet, either in a float or as a horizontally translatable position indicator, represents the position of a monitored quantity of liquid or monitored position of an object of interest.
The use of the magnetostrictive principle involves the creation of an initial current pulse within a magnetostrictive wire that interacts with the magnetic field created about the wire at the permanent magnet's location causing a torsional disturbance. Because a torsional wave is essentially an acoustic wave, its speed of propagation is sufficiently slow and measurable that a direct relationship can be established between the time it takes for the wave to travel and the distance traveled. When the end points of the wire are known and the initial start time for the torsional pulse is known, the detection of the torsional pulse at an end point of the wire will provide a time value directly related to the distance traveled by the torsional pulse. This distance is then used to determine the level of a liquid within a tank from the top to the bottom of the tank (and therefore to determine the volume of liquid remaining in the tank) or the position of a machine tool as it works an object of concern.
Use of the magnetostrictive principle in liquid level and positioning applications will typically take one of two forms. In each case, the principle involves the "interrogation" of the magnetostrictive wire with either an initial torsional pulse or an initial current pulse. In one method, a current pulse is directed through the magnetostrictive wire inducing a magnetic field around the wire. This electrically induced magnetic field interacts with the magnetic field established by the permanent magnet that is placed around the magnetostrictive wire. The interacting magnetic fields cause torsional forces that are translated into an acoustic/torsional wave in the magnetostrictive wire. Because the current pulse is essentially instantaneous (traveling at the speed of light), the start of the torsional wave can be considered as the start in time of the current pulse in the magnetostrictive wire. The torsional pulse, however, travels at the speed of sound in the wire which is much slower than the speed at which the current pulse travels and thus the time delay between the initial current pulse (the start of the torsional pulse) and the reception or detection of the torsional pulse at an end point of the magnetostrictive wire is measurable.
An alternative to the above method of utilizing the magnetostrictive principle involves imparting a torsional wave to the magnetostrictive wire, allowing it to travel down the wire to the point that it encounters the magnetic field created by the permanent magnet. The torsional motion of the wire within this magnetic field induces a current within the wire which immediately travels to the wire end points in a manner that can be detected. In either case, there is an exchange between electrical energy and mechanical energy and the appropriate detection of either electrical energy or torsional wave energy in response.
Various devices and systems that have been addressed in the past are primarily directed towards improved means for either measuring the time delay between the current pulse and the acoustic pulse or identifying and distinguishing the acoustic pulse from extraneous acoustic noise elements that occur. Much attention has been paid to various types of sensors that can accurately distinguish the particular torsional pulse of interest from other acoustic and electrically induced wave elements within the magnetostrictive wire.
Other patents and disclosures in the prior art address various means for absorbing one of the two torsional waves that travel outward from the point of interaction with the permanent magnetic field. When a current pulse is imparted to the wire, a torsional wave is initiated at the point where the wire intercepts the magnetic field of the permanent magnet and propagates in both directions along the magnetostrictive wire, although typically only one direction represents a distance that is of concern. In the opposite direction, it is generally desirable that the torsional wave be dampened and/or eliminated so that it is not finally reflected from an opposite end point of the magnetostrictive wire back to the sensor end of the magnetostrictive wire and confused with the initial wave.
The sensor that is typically at the top of or at one end of the magnetostrictive wire can be and has been described as being comprised of many different materials. Electromagnetic sensors and piezoelectric sensors have been used successfully to detect current pulse at the end point in the magnetostrictive wire and the arrival of the torsional wave, respectively.
Piezoelectric sensors have been shown to be most useful in accurately detecting and discriminating the arrival of the torsional pulse with an accuracy sufficient for many applications. In one configuration described in more detail below, two small plates of piezoelectric material, typically made up of one of a number of ceramic piezoelectric compositions, are bonded to diametrically opposed surfaces of the magnetostrictive wire. The opposite faces of these plates are then bonded to a housing or other stable mounting structure such that torsional movement of the magnetostrictive wire can be detected. FIG. 2a and 2b, described in more detail below, show two alternative prior art means for implementing conventional piezoelectric torsional wave sensors.
Piezoelectric sensors are useful in both methods of magnetostrictive application described above. The piezoelectric crystals can be stimulated by an electric current to produce a torsional wave in the magnetostrictive wire or they may be used to convert a torsional wave detected in the wire into an electric signal that is used to terminate the measured time period of concern.
The normal operational mode of piezoelectric sensors as they are utilized in such applications is in a shear mode, but certain longitudinal extensions can also be utilized. In some configurations, the piezoelectric materials are physically oriented and electrically connected so as to null certain common mode vibrations in the magnetostrictive wire, but for such nulling to be effective, the sensitivities of the two plates in both the primary axis and off axis directions must be matched closely. In addition to this, the housing to which the ends of the piezoelectric plates are attached must be either rigid or have a high inertial mass and the plates must be precisely aligned to avoid introducing false signals and undesireably enhancing an off axis vibrational response. There is, therefore, a high degree of calibration, matching, and alignment required with conventional piezoelectric elements, that is both time consuming and expensive, but necessary in order to provide a detector of sufficiently accurate character.
A number of additional problems are often encountered in the systems described in the prior art, some of which utilize piezoelectric elements and some of which do not. Many problems derive from trying to identify the point on the torsional wave that is considered to be the trigger point or the end of the time period to be measured. As indicated above, some amplification of the torsional wave caused by external factors can result in the trigger point being detected inaccurately. There is some necessity in many systems, therefore, to isolate and/or filter the torsional wave as it travels in the wire so as to allow the sensor a "cleaner" wave form for detection.
The sensitivity of a magnetostrictive wire based detector, however, is only partially related to the sensitivity of the sensor it incorporates. Much of the sensitivity and the quality of a liquid level measuring device or a position measuring device relates to the proximity of the permanent magnet to the magnetostrictive wire and the resultant magnetic coupling between the magnet and the current pulse. While sensitivity factors favor placing the permanent magnet close to, indeed surrounding the wire in many cases, accessibility and maintenance factors favor placing the sensor away from the liquid or object being measured and thus away from the permanent magnet that must be intimate to the liquid or object. The farther the permanent magnet is from the magnetostrictive wire, however, the more significant are various electromagnetic and mechanical anomalies in the system and the more difficult it is for the transducer to distinguish the torsional wave of importance from anomalies and background noise. Ideally a magnetostrictive distance or level measuring apparatus would have a combination of improvements that would together create a sensor with improved sensitivity without greatly increasing the cost of the apparatus and would permit greater emphasis on accessibility and maintenance.
It would be desirable to have a magnetostrictive wire based level or position transducer that utilizes a sensor element capable of distinguishing a specific torsional wave or current pulse, as well as a structural arrangement that allows greater mechanical isolation between the magnetostrictive wire and the magnetic position indicator. It would further be desirable to provide such a magnetostrictive wire based system that functions on a low power current source, provides easy access and maintenance, and reduces the time and expense necessary for installation and calibration.